# RF injection scanning tunneling spectroscopy of a superconducting NbSe2 surface

## TL;DR

This paper uses a specialized scanning tunneling microscopy technique to study superconducting properties of NbSe2 surfaces and how they respond to radio frequency signals.

## Contribution

The study introduces RF injection scanning tunneling spectroscopy to observe quasi-particle behavior and phase-specific superconducting properties in NbSe2.

## Key findings

- Quasi-particle states split and widened with increasing RF power, explained by the Tien-Gordon model and Bessel function behavior.
- The superconducting gap decreases at the boundary between 2H and 1T phases of NbSe2.
- RF-induced energy separation shows different coefficients in 2H and 1T phases due to varying dielectric and shielding properties.

## Abstract

NbSe2 is a transition metal dichalcogenide with a two-dimensional nature, showing superconducting (SC) and charge density properties. Moreover, a 1T phase can be made in the film, which has a different property from the bulk dominant 2 H phase, in which Mott insulator behavior is actively discussed. We observed the surface with STM at 400 mK and injected RF signal (1 GHz and 15 Hz) at the tunneling junction. We detected two quasi-particle (QP) states at the end of the SC gap, which split with the increase of the RF power specified by the electric field at the tunneling junction, VAC. A previous STM experiment with 65 GHz RF on a vanadium surface observed multiple replicas of the QP peak. However, our experimental result using 1 GHz RF shows a widening of QP features with two enhanced peaks shifted by ~ ± eVAC from the original QP positions. The behavior was well reproduced by a simulation using a well-known Tien-Gordon model, whose results indicate that the disappearance of multiple peaks is due to the low frequency of the RF signal. In addition, two enhanced peaks at ~ ± eVAC are deduced from the Bessel function behavior. The energy shift from the original peak linearly changes with eVAC. We apply this technique to examine the property change at the domain boundary of the 2 H and 1T phase of the NbSe2 surface. We found the superconducting gap decreases when we move the tip from the 2 H domain into the 1T domain. Moreover, the injection of RF splits a QP peak into two enhanced peaks, whose energy separation is linear with the electric field at the RF generator for both phases. However, the linear energy separation with VAC shows different coefficients between the 2 H and 1T phases. We conclude that the different coefficient is due to the change of actual VAc on the two domains originating from a different dielectric constant and shielding efficiency for the electric field of RF.

The online version contains supplementary material available at 10.1038/s41598-025-07203-2.